1. Field of the Invention
The present invention relates to an optical-axis deflection type laser interferometer that includes a reference ball serving as a measurement reference, a retroreflecting means disposed at an object to be measured, a laser interferometer length measuring apparatus that outputs a measurement value according to an increase or a decrease in distance between the apparatus and the retroreflecting means, and a rotational mechanism that rotates a beam projected from the laser interferometer length measuring apparatus around the reference ball, the laser interferometer measuring a distance between the laser interferometer and the retroreflecting means that makes an optical axis of an outgoing beam projected from the laser interferometer length measuring apparatus mounted on the rotational mechanism and an optical axis of a return beam returned to the laser interferometer length measuring apparatus parallel to each other, based on center coordinates of the reference ball. The present invention relates to a calibration method of the laser interferometer, relates to a correcting method of the laser interferometer, and relates to a measuring method of the laser interferometer. More specifically, the present invention relates to an optical-axis deflection type laser interferometer that can secure traceability by the length standard without using a special device, relates to a calibration method thereof, relates to a correcting method thereof, and relates to a measuring method thereof.
2. Description of the Related Art
An optical-axis deflection type laser interferometer (also called a “tracking laser interferometer”) is known. This laser interferometer operates in the following way. As shown in FIG. 1, a laser beam (hereinafter, referred to as a “measurement beam”) 22 is projected from a laser interferometer length measuring apparatus 20 toward a retroreflecting means (also called a “retroreflector”) 12 disposed at a to-be-measured object 10, and is reflected in the backward direction by the retroreflecting means 12. The displacement of the retroreflecting means 12 is detected by use of the interference of the laser beam reflected therefrom, and a tracking operation is performed by use of a change in position of the optical axis of the measurement beam 22 by a two-axis rotational mechanism 30. In FIG. 1, reference numeral 24 designates a light source, and reference numeral 32 designates a deflective plane mirror of the two-axis rotational mechanism 30.
According to this optical-axis deflection type laser interferometer, based on information about the return beam returning from the retroreflecting means 12 that serves as a target, the distance from the apparatus to the target can be measured by the laser interferometer length measuring apparatus 20 with high accuracy.
However, the optical-axis deflection type laser interferometer has technical difficulties in making the precision of the two-axis rotational mechanism 30 as high as the precision in measurement of the laser interferometer. Especially, as in the optical system shown in FIG. 1 in which the direction of the measurement beam 22 is deflected by, for example, the plane mirror 32 attached to the two-axis rotational mechanism 30, the two-axis rotational mechanism 30 has a precision limit, and, in addition, it is difficult to allow a beam to accurately strike the rotational center of the plane mirror 32. These make it difficult to achieve high precision.
Therefore, as shown in FIG. 2, European Patent EP0919830A2 (hereinafter, referred to as Patent Document 1) discloses that a relative displacement between the surface of a reference ball 34 disposed at the center of the two-axis rotational mechanism 30 and the laser interferometer length measuring apparatus 20 is measured with a laser beam 26 projected in the exactly opposite direction with respect to the target (12), in addition to the measurement of the retroreflecting means 12 serving as a target. In FIG. 2, reference numeral 36 designates a carriage used to mount the laser interferometer length measuring apparatus 20 on the two-axis rotational mechanism 30, and reference numeral 40 designates a support used to fasten the reference ball 34.
However, the optical system disclosed by Patent Document 1 has a disorder of an interference wavefront or a reduction in light quantity with respect to a rotational error (i.e., runout) in a direction perpendicular to the optical axis. Additionally, it has become apparent that the optical system is not necessarily robust in respect of a change in the optical path length. Therefore, high-cost specifications are required for the rotational accuracy of the two-axis rotational mechanism 30. Additionally, the problem of the light quantity ratio of the laser interferometer length measuring apparatus 20 is included therein. In more detail, the laser interferometer length measuring apparatus 20 measures a phase difference between a reference beam whose optical path is unchangeably fixed there inside and a measurement beam 22 that travels toward the target (12) and that returns backwardly therefrom. Therefore, if there is a great difference in light quantity between the reference beam of light and the measurement beam of light, the contrast of interference fringes cannot be sufficiently secured, and, as a result, interference measurement cannot be performed with high accuracy. However, in the optical system disclosed by Patent Document 1, the measurement beam is first projected and reflected to and from the reference ball 34, and is then projected and reflected to and from the target (12). The light quantity of the measurement beam that has returned in this way is remarkably attenuated, and hence a difference in light quantity with respect to the reference beam that is generally regarded as not being easily attenuated causes a large problem.
On the other hand, the present applicant proposes an optical system shown in FIG. 3 in Japanese Published Unexamined Patent Application No. 2007-57522 (not yet published at the priority date thereof: hereinafter referred to as Patent Document 2). In this prior invention, a motion error of the two-axis rotational mechanism 30 with respect to the reference ball 34 is measured with a displacement gauge 50 separately mounted on a carriage 38 formed integrally with the carriage 36, not with a measurement beam of the laser interferometer length measuring apparatus 20. According to this prior invention, a highly accurate optical-axis deflection type laser interferometer can be, of course, realized with reference to the reference ball 34, and the precision requirement against the two-axis rotational mechanism 30 can be greatly eased without sacrificing robustness with respect to an error component in the direction perpendicular to the optical axis. Additionally, according to this prior invention, the measurement beam of the laser interferometer length measuring apparatus 20 is projected only onto the target (12), and hence the disadvantage of the attenuation of the measurement beam that becomes a problem in the technique of Patent Document 1 is greatly reduced.
However, in this prior invention, a motion error in the direction of the optical axis of the two-axis rotational mechanism 30 with respect to the reference ball 34 is detected, and hence there is a need to additionally provide the displacement gauge 50. Since the optical-axis deflection type laser interferometer has been expected to perform highly accurate measurement that is traceable to the length standard, the technique to secure the traceability of the displacement gauge 50 has been expected to be achieved.